api_extensions: introduce idmapped_mounts_v2 api extension

doc/api-extensions.md

@@ -138,4 +138,9 @@ Whether the seccomp notify proxy sends a long a notify fd file descriptor.
 
 ## idmapped\_mounts
 
-Whether this LXC instance can handle idmapped mounts.
+Whether this LXC instance can handle idmapped mounts for the rootfs.
+
+## idmapped\_mounts\_v2
+
+Whether this LXC instance can handle idmapped mounts for lxc.mount.entry
+entries.

src/lxc/api_extensions.h

@@ -42,6 +42,7 @@ static char *api_extensions[] = {
 	"seccomp_notify_fd_active",
 	"seccomp_proxy_send_notify_fd",
 	"idmapped_mounts",
+	"idmapped_mounts_v2",
 };
 
 static size_t nr_api_extensions = sizeof(api_extensions) / sizeof(*api_extensions);